The present invention relates to a method and apparatus for synchronizing a receiver to a timing and carrier frequency of a communication system, specifically to an initial synchronization of a wireless receiver in a code division multiple access (CDMA) system with time-division duplex (TDD) for payload data transmission.
In CDMA, each user is assigned a unique code sequence it uses to encode its information-bearing signal. The receiver, knowing the code sequence of the user, decodes a received signal after reception and recovers the original signal. This is possible since the cross-correlations between the code of the desired user and the codes of other users are small. Since the bandwidth of the code signal is chosen to be much larger than the bandwidth of the information-bearing signal, the encoding process enlarges or spreads the spectrum of the signal and is therefore also known as spread-spectrum modulation. The rate of the CDMA signal is called the chip rate, wherein one chip denotes one symbol when referring to spreading code signals. After transmission of the CDMA signal, the receiver typically uses coherent demodulation to despread the CDMA signal, using a locally generated code sequence. To be able to perform the despreading operation, the receiver must not only know the code sequence used to spread the signal, but the codes of the received signal and the locally generated code must also be synchronized. This synchronization must be accomplished at the beginning of the reception and maintained until the whole signal has been received.
Time-Division Synchronous Code-Division Multiple Access (TD-SCDMA) and TD-SCDMA System for Mobile (TSM) are 3rd generation (3G) and 2.5th generation (2.5G) standards for mobile communication, respectively, for which products are currently under development. These standards support broadband packet-based transmission of text and multimedia data—such as audio, video and digitized voice—at a high data rate. The physical layer of both standards is widely identical and is based on CDMA with TDD for payload data transmission. This transmission standard specifies that each base station (BTS) transmits a unique 64 chip synchronization sequence SYNC to help the terminal device or user equipment (UE) in frame, frequency, and block synchronization.
For spectral efficiency, a target for cellular deployment is a frequency reuse factor equal to one, like it is done in other CDMA systems. This means that neighboring cells use the same carrier frequency and therefore cause mutually interfering signals at the UE. It is one special feature of TD-SCDMA that BTSs are frame-synchronized so that the received SYNC signal portion is corrupted by slightly time shifted interfering SYNC signals from neighboring cells, which degrades timing results and frequency estimates obtained from simple state-of-the-art correlation algorithms. As a further complication, all channels suffer from multipath propagation.
The carrier frequency is currently estimated by exploiting the SYNC of a single BTS by a correlation technique, which is close to optimum in non-dispersive channels with white noise, i.e., without interference. However, simple correlation degrades in dispersive (i.e., multipath) channels and breaks down in strong interference.
In “Frequency Estimation for the Downlink of the UMTS-TDD Component”, Michele Morelli et al, IEEE Transactions on Wireless Communications, pp. 554-557, vol. 1, no. 4, October 2002, an estimator is proposed which solves the multipath problem by using least-squares (LS) channel estimation (CE) based on the transmitted SYNC signal. The estimator then computes the frequency estimate from phase differences between corresponding channel taps estimated from different portions of SYNC. The authors described their algorithm for the training sequence of the high chip rate TDD option in the 3GPP (3rd Generation Partnership Project) specification TS 25.223, i.e., not for TD-SCDMA, and estimation is based on the training signal from one single BTS, only.
The start of the block is usually detected by a correlation of detected SYNC phases with the specified phase sequence, in a so-called matched filter, which indicates the start of the block if the output is large. Commonly, the correlation window has fixed size and a threshold is used to decide the block start from the correlation result.
However, because TD-SCDMA is a CDMA system, high frequency efficiency will demand deployment of BTSs with frequency reuse factor equal to one in the long run. This means strong co-channel inter-cell interference, which is not sufficiently suppressed by the relatively short SYNC training sequences provided for frame, frequency, and block synchronization. All state-of-the-art frequency estimators strongly degrade in the presence of inter-cell interference and will not allow reliable synchronization in cellular systems with a reuse factor equal to one. Also the SYNC phase detection for block synchronization can suffer from interference.